Acceleration-deceleration pneumatic device

ABSTRACT

A SINGLE PNEUMATIC CYLINDER IS DESCRIBED WHEREIN GAS PRESSURE APPLIED TO ONE FACE OF A PISTON ACCELERATES THE PISTON FOR A PORTION OF A STROKE, WHEREUPON THE PISTON REACHES A GAS RELIEF REGION WHICH PERMITS GAS TO FLOW TO THE OPPOSITE SIDE OF THE PISTON WHERE IT IS COMPRESSED TO DECELERATE THE PISTON ORIFICE MEANS ARE PROVIDED FOR LIMITING GAS FLOW FROM THE SECOND END OF THE CYLINDER FOR CONTROLLING DECELERATION TO BRING THE PISTON TO A STOP AT THE END OF THE STROKE WITHOUT SUBSTANTIAL IMPACT OR REBOUND AND WITHOUT CEASING THE FLOW OF PRESSURIZING GAS USED FOR ACCELERATING THE PISTON.

United States Patent [72] Inventor Berge Hagopian 1,490,633 4/1924Peters 91/399 Costa Mesa, Calif. 2,463,537 3/1949 Hoor et a1... 91/416[21] App1.No. 763,688 2,569,504 10/1951 Thierry 91/399 [22] Filed Sept.30, 1968 2,703,558 3/l955 Wilcox 9l/4l6 [45] Patented June 28, 19712,746,425 5/1956 Schafcr 91/416 [73! Assignee North American RockwellCorporation Primary Examiner Paul E Muslousky Attorneys-William R. Lane.Allan Rotlhenberg and Richard [54] ACCELERATION-DECELERATION PNEUMATICSeibel DEVICE 2 C1 2D Fi aims rawmg gs ABSTRACT: A single pneumaticcylinder is described U-S. ..i .v wherein gas res ure to one face of apiston ac 92/164, 92/253, 92/258 celerates the piston for a portion of a:stroke, whereupon the [51] ll". piston gaches a gas relief region whichermits gas to flow to 1 9/00 the opposite side of the piston where it iscompressed to [50] Field of Search ..91/399, 416 decdcrate the piston Oifi means are provided f Hmiting (cursory); 92/253 (cursory), 253 y) gasflow from the second end of the cylinder for controlling deceleration tobring the piston to a stop at the end of the [56] References cued strokewithout substantial impact or rebound and without UNITED STATES PATENTSceasing the flow of pressurizing gas used for accelerating the 3,175,4743/1965 Eickmann 92/253 piston.

14 43 Y 37 34 ll [3 42 ACCELlERATION-DECELERATION PNEUMATIC DEVICEBACKGROUND The invention described herein was made in performance ofwork under a NASA contract and is subject to the provisions of theNational Aeronautics and Space Act of 1958, Public SUMMARY OF THEINVENTION Therefore, in the practice of this invention according to apreferred embodiment, there is provided a single cylinder having apiston slidably movable therein with gas relief means within thecylinder for permitting gas flow from one end of the cylinder to theother end thereof at a selected piston location. Means are provided forintroducing pressurized gas for accelerating the piston and, after gasflows through the gas relief means, the piston compresses the gas in achamber having having limited gas flow for deceleration of the pistonwithout substantial rebound or impact shock.

Attendant advantages of this invention will be readily appreciated asthe same becomes better understood by reference to the followingdetailed description when considered in connection with the accompanyingdrawings where:

FIG. 1 comprises a perspective view of a pneumatic device as provided inpractice of this invention according to a preferred embodiment; and

FIG. 2 comprises a longitudinal cross section of the device of FIG. 1.

Throughout the drawings like reference numerals refer to like parts.

DESCRIPTION FIG. 1 illustrates a pneumatic acceleration-decelerationdevice incorporating the principles of this invention. As illustrated inthis embodiment there is provided a generally cylin drical housinghaving a rectangular flange 11 in which bolt holes 12 are provided formounting the flange on a supporting structure. A rectangular boss 13 isprovided on the housing and includes a threaded opening 14.

A first end of the housing 10 is closed by a cap 16 screwed into thehousing and sealed thereto by a a conventional O-ring 17 as seen in thethe cross section view of FIG. 2. The other end ofthe housing is closedby a cap 18 screwed into the hous ing by spanner holes (not shown) andsealed thereto by an O- ring 19. A gland 21 is seated in the housing forcontaining a dynamic seal 22 against the end cap 18 and against a pistonrod 23 which is slidably movable within the housing. The end cap 18 andgland 21 not only capture the seal 22, but also provide bearing supportfor the piston rod 23. The dynamic seal 22 may, for example, be a springloaded rubber seal such as is commercially available from Bal SealEngineering Company, La Habra, California.

The piston rod 23 has a threaded end 24 extending from the housing forconnection to an object to be actuated by the pneumatic device. Nearerthe other end of the piston rod 23 is a flange 26 having a smallclearance from an inside cylindrical bore 27 in the housing 10. A piston28 is seated against the flange 26 by a retainer 29 which is secured tothe piston rod 23 by a nut 31 threaded thereon. A hexagonal portion 32on the end of the piston rod 23 permits tightening of the nut 31 and anyconnection to the threaded portion 24.

A lip 33 on the flange 26 overlaps an end of the piston 28 to hold inplace a circular rubber seal 34 having a generally U- shaped crosssection. A plurality of radially extending holes 36 are provided throughthe lip 33 to permit gas to freely enter the seal in the open part ofthe U. Similarly, a lip 37 is provided on the retainer 29 for holdingcaptive a similar U-shaped cross section seal 38 facing in a directionopposite that of the seal 34. A plurality of radially extending holes 39are provided through the lip 37 to permit gas to freely enter the openportion of the U-shaped seal. Seals 34 and 38 are commercially availableunder the trademark Omniseal," from Airoquip Corporation, Burbank,California.

In operation, gas pressure within the open part of the U- shaped sectionof the seal causes expansion thereof so that good sealing is obtainedbetween the piston 28 and the cylindrical bore 27 in the housing. Thepair of oppositely facing seals 34 and 38, respectively, are employed sothat the piston operates equally well with higher pressure gas on theside facing the cap 16 as compared with the side facing the cap 18 orvice versa. The significance of this will be appreciated hereinafterwhen it is recognized that one seal 38 is operable during accelerationof the piston and the other seal 34 is operable during deceleration ofthe piston. The seals 34 and 38 are not only tightly sealed by gaspressure therein, but also are retained captive by the lips 33 and 37,respectively, so that they do not blow out" when the piston is out ofcontact with a surrounding wall. The piston 28 is also statically sealedto the piston rod 23 by a conventional O-ring 41.

Within the housing 10 there is a cylindrical bore 27 as hereinabovedescribed. The bore 27 continues as a single cylindrical bore 27' on theopposite side of an enlarged portion 42 in the bore of the housing,intermediate the ends thereof. The enlarged portion 42 includes a pairof oppositely facing conical tapered regions 43 extending between thesingle, constant diameter cylindrical bore 27, 27' and the enlargedportion 42. In a preferred embodiment the angle between the conicalportions 43 and the cylindrical bore 27, 27 is about 15 in order obtainoptimum operation ofthe seals 34 and 38 without cutting thereof. Propergas flow around the piston length of stroke is provided in a minimumdistance during operation by the gently sloping, conical surfaces 43 andthe enlarged portion 42. The seals 34 and 38 are gradually and uniformlypressed into position in the piston 28 by the surfaces 43 as ittraverses the gas relief portion 42 and a large number of cycles ofoperation of the piston with this arrangement demonstrates that noappreciable seal wear is obtained.

At one end of the housing 10 a plenum 44 is provided communicating withthe gas inlet port 114 for introducing highpressure gas to one side ofthe piston for causing movement of the piston and piston rod.

An orifice 46 is provided through the end cap 15 and one or more holes47 are provided through the gland 21 so that the orifice 46 is in goodfluid communication with the interior of the housing. The gland 21 maybe keyed to the cap 18 so that the orifice 46 and a hole 47 are alignedupon assembly of the device; however, alignment is unnecessary since achamfered corner on the gland provides a gas flow path to the orifice.The orifice is provided for bleeding gas out of the cylindrical bore 27'at a controlled rate. This rate is readily predetermined by changing theorifice diameter (area) and a plurality of caps having different orificediameters are readily employed for varying the orifice size to providecontrolled deceleration in a selected situation. Rather than a pluralityofcaps, it will be apparent that orifice inserts may be provided in thecap or in the side of the housing for obtaining various orifice sizes.Similarly, an orifice plus a needle valve or the like may be employedfor control of gas flow.

It should be noted that in addition to gas flowing from the interior ofthe housing through the orifice 46 there is also some gas leakage pastthe dynamic seal 22 around the piston rod and the seal 34 on the piston.It is found in some situations that this virtually unavoidable gas flowat the dynamic seals is sufficient to provide an acceptabledeceleration. A dynamic test of a particular sealing arrangementindicates the equivalent orifice area of the seal and the orifice issized accordingly to bring the gas pressure on the compression side ofthe piston to substantially the same pressure as the gas on the drivingside of the piston only at the end of the stroke so that the pistonstops without substantial impact or rebound, the energy of the movingmass is irreversibly transferred to the gas compressed beneath thepiston.

In operation the piston is positioned in the housing in the locationillustrated in FIG. 2 either by hand or by some pneumatic mechanism (notshown) for providing a return stroke. When a power stroke is desiredpressurized gas is rapidly admitted to the plenum 44 to bear on the faceof the piston and cause acceleration of the piston rod along the firstpart of the stroke while the piston is in sealing engagement with thecylindrical bore 27. When the piston reaches the enlarged portion 42 ofthe bore in the housing, gas is free to pass around the piston throughthe enlarged portion from the gas plenum 44 to the portion of thecylindrical bore 27' on the other side of the piston thereby equalizingpressure on both sides of the piston.

Inertia of the moving piston and any mass attached to the piston rodcause the piston to travel rapidly through the enlarged portion 42 andbring the piston into sealing engagement with the cylindrical bore 27 toagain form a substantially closed chamber behind the piston. The seals34 and 38 on the piston may expand as the piston reaches the enlargedportion 42. The lips 33 and 37 keep the seals from being dislodged,however. The conical sloping portions 43 gradually and uniformlycompress the seals and bring them into sealing engagement with the bore27'. It should be noted that at the instant the piston seals against thecylindrical bore 27 the pressure on both sides of the piston issubstantially identical.

As the piston continues to travel toward the closed end of the housingthe gas in the chamber ahead of the piston is compressed, therebyraising the pressure above that on the driving side of the piston andgenerating a decelerating force against the piston to bring it and anymass attached to the piston rod to a stop. If the chamber in the housingbehind the piston were completely closed so that no gas could escapetherefrom the increasing gas pressure would stop motion of the piston,however the energy stored in the gas would actually cause a reversal ofmotion to the cause the piston to rebound with an accelerationsubstantially the same as the deceleration. In order to prevent anysubstantial rebound, the orifice 46 is provided so that there is a meansfor bleeding gas out of the substantially closed chamber at a controlledrate, both for preventing rebound by venting the compressed gas andproviding a controlled deceleration of the piston. It will, of course,be recog nized that if the orifice is too large insufficient compressionof a gas behind the piston is obtained and the piston may havesubstantial impact load so it bottoms against the gland 2]. Properorifice sizing is therefore important to achieve an appropriatedeceleration without impact load at the end of the stroke or rebound ofthe piston.

The orifice area for controlled deceleration can be approximated on thebasis of the kinetic energy of the moving piston and any objectconnected thereto at the beginning of the com pression or decelerationportion of the stroke, the stroke length over which deceleration isdesired (i.e., the maximum deceleration), and the working pressure ofthe gas. The kinetic energy is determined by the mass of the piston andany object connected thereto and its velocity (K.E.=%Mv or kineticenergy equals one-half mass times velocity squared). The velocity isdependent on the gas pressure, piston area, stroke length and anyresisting forces (including the mass of the piston and any objectconnected thereto)(v=as or velocity squared equals two timesacceleration times stroke; and a=PA/M or acceleration equals drivingpressure times piston area over mass). The work done on the gascompressed by the piston must balance the kinetic energy of the movingmass at the end ofthe stroke. Assuming an adiabatic compression, thework is dependent on the final and initial energy contents of the gas(Work=(P,V,-P,V,)l l--k or work is equal to the final pressure timesvolume less the initial pressure times volume, all over l-k where k=l .4for adiabatic compression). The initial pressure, volume and work areknown, and assuming that there is no gas leakage, the final pressure andvolume are found for full energy absorption in the compressed gas. Thispressure, P is used to find an average pressure behind the piston whichapproximates the numerical average of the starting (and ending) pressureand the maximum pressure P,. The weight of gas behind the piston isknown from the volume and density and the time of the compression strokeis known from the deceleration and stroke, therefore the average massflow rate is found. If the gas pressure outside the orifice is less that0.53 times the pressure inside the orifice the velocity of gas throughthe orifice is a known terminal value (e.g., 1,020 ft./sec. for air) andthe required orifice area is found from the mass of gas that must flowthrough the orifice during the time of the stroke.

It is apparent that the determination of an orifice area as set forth isonly approximate and that a precise finding would require a solution ofintegral equations accounting for the changing mass flow through theorifice with changing gas density due to changing pressure, and thechanging volume of the trapped gas. it appears that an orifice with aneffective area about two-thirds of that found by the above technique iseffective. It should be noted that an approximate determination oforifice area is quite satisfactory since some leakage is virtuallycertain around the dynamic seals and the actual orifice area for aparticular application is best found empirically, commencing with anorifice area estimated as pointed out herein.

In a specific application of an acceleration-deceleration device it wasdesired to rapidly move a l50pound mass through a distance of about 2%inches with a friction force of about 720 pounds during the firstthree-fourths inch of travel. For this purpose four pneumaticacceleration-deceleration devices were employed to provide symmetricalloading. Each device had a total stroke of 2% inches a piston diameter.of 0.738 inch. An operating pressure of 750 p.s.i.g. was used to drivethe piston through the first three-fourths inch of stroke and thispressure was applied at the beginning of the stroke and maintained untilthe end of the full 2% inch stroke. The velocity at the end of the powerstroke was about 3.8 feet per second. The piston (and mass attachedthereto) traveled about three-eighths inch through the enlarged portionof the cylinder bore between the positions where one piston seal leftengagement with the cylinder wall and the other seal engaged thecontinuation of the cylinder wall. The enlarged portion had a diameterof seven-eighths inch which permitted unrestricted gas flow. During thistravel little change in velocity occurred and gas flowed freely aroundthe piston to equalize pressure on both sides thereof at about 750p.s.i.g, The total deceleration stroke was about 1% inch long,compressing gas behind the piston and forcing it out through the relieforifice to bring the piston to a stop without substantial impact orrebound. A maximum acceleration of about 10 gs was mea sured on thepower and deceleration portions of the stroke. An orifice with aneffective area of a little less than 0.0006 square inch including theleakage around the dynamic rod seal and piston seal provided excellentresults with high reliability and reproducibility.

It will be apparent that many modifications and variations can be madein a pneumatic acceleration-deceleration device without departing fromthe principles of this invention. Thus, for example, in addition to theabove suggested variations in the orifice, the piston arrangement couldbe modified, the piston rod could extend from the opposite end of thehousing to provide a pulling" rather than pushing motion, other mountingarrangements can be provided, and the like.

lclaim:

1. A pneumatic acceleration-deceleration device comprisin a housinghaving a cylindrical bore;

a piston slidably movable in said housing;

a dynamic seal for sealing said piston to said cylindrical bore forsubstantially preventing gas flow therearound, while said seal is seatedon said cylindrical bore;

gas relief means intermediate the ends of said bore for dividing saidbore into a first cylindrical region towards one end and a secondcylindrical region towards the other end, whereby gas can flow from saidone end to said other end when said piston is positioned adjacent saidgas relief 5 means;

means for introducing pressurized gas into said one end for acceleratingsaid piston toward said other end;

orifice means for providing limited gas flow from said other end ofsaidcylindrical bore whereby gas in said other end of said cylindrical boreis compressed by said driven piston for gradually decelerating saidpiston without sub stantial rebound or impact;

said gas relief means comprising a first conical tapered enmeans on saidpiston for preventing said dynamic seal from being dislodged when saidpiston is positioned adjacent said gas relief means;

said dynamic seal comprising a first sealing member arranged to preventgas flow from said one end to said other end of said cylindrical borewhen the pressure in the one end exceeds the pressure in the other end;and a second sealing member arranged to prevent gas flow from said otherend to said cylindrical bore when the pressure in said other end exceedsthe pressure in said one end;

said housing comprising a cylindrical housing body; and an end cap forsaid housing body; and

said orifice means being in said end cap so that interchanging of endcaps can effect a change in effective orifice area.

2. A pneumatic acceleration-deceleration device as defined in claim 1wherein said piston comprises:

a piston rod extending through said end cap;

a flange on said piston rod;

a lip on said flange;

a retainer removable secured on said rod;

a lip on said retainer, the lip on said flange and the lip on saidretainer facing towards each other; and

a piston member between said flange and said retainer, said pistonmember cooperating with said lips for preventing said first and secondsealing, members from being dislodged when said piston is adjacent saidgas relief means.

